In recent years, with reduction in size and weight of electronic devices such as cellular phones and notebook personal computers, secondary batteries used as power sources for these devices are required to have higher capacities. In response to such a requirement, non-aqueous electrolyte secondary batteries, which have the potential for higher energy densities, are widely used. A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator interposed therebetween, and a non-aqueous electrolyte. The positive electrode, the negative electrode, and the separator are wound together, forming an electrode group.
The positive electrode includes a current collector, and a positive electrode material mixture layer formed on a surface of the current collector. The positive electrode material mixture layer includes positive electrode active material particles, a binder, and, if necessary, a conductive material. A lithium-containing transition metal oxide, such as LiCoO2, LiNiO2, LiNi1-x1Cox1O2, where 0<x1<1 is used as the positive electrode active material.
In order to achieve a higher capacity of non-aqueous electrolyte secondary batteries, it has been studied to densely pack positive electrode active material particles in the positive electrode material mixture layer, thereby to increase the active material density. However, in winding a positive electrode in which the active material density is high, breakage of the positive electrode such as rupture of the current collector or chipping and cracks in the positive electrode material mixture layer occurs easily due to the stress of winding. Therefore, suppression of such breakage of the positive electrode has been studied.
Patent Literature 1 proposes that the electrode material mixture layer be provided with recesses at predetermined intervals, thereby to divide it into a plurality of regions. Patent Literature 1 discloses that this makes it possible to curve the electrode to a great extent without causing breakage.
Patent Literature 2 proposes that the electrode material mixture layer and the current collector be allowed to separate from each other easily at the interface therebetween. It is disclosed that the stress of winding causes a slight separation at the interface, and as a result, the breakage of the electrode can be prevented. Patent Literature 2 further proposes that the peeling strength at the interface between the current collector and one of the electrode material mixture layers which is provided on the inner side of the current collector be set smaller than that between the current collector and the other one of the electrode material mixture layers.
Patent Literature 3 proposes that the binder be included in the electrode material mixture layer such that the concentration of the binder in an area around the center is 50 to 90% of that in an area near the current collector. Patent Literature 3 discloses that this can decrease the amount of the binder, without impairing the adhesion between the current collector and the electrode material mixture, and thus can improve the charge/discharge characteristics.